Allows a malicious application to add bytes of code to the APK or DEX formats used by Android applications without affecting the application’s signature. In other words, a scumbag could pack an app with malicious instructions, and still have it read by Android as a trusted piece of software.

Android 5+

NOTE: I did some research and this should be fixed in security patch Dec 5, 2017 which should reach supported Google devices (such as Pixel) in about 2 weeks. For devices that do not get their patches directly from Google, it will take longer.